Uranium isotope separation process following the molecular laser process

ABSTRACT

The process of the present invention provides a gas steam of a mixture of UF 6  isotopes and an inert gas, e.g. nitrogen, which is adiabatically expanded through a nozzle into a laser light excitation zone and photodissociated to form U-235 enriched UF 5 . 
     After the gas stream has passed through the laser light excitation zone, XeF 6  is fed into the process gas stream so the the xenon hexafluoride remains protected against the dissociating radiation. The XeF 6  may be mixed with the same inert gas that is employed for the adiabatic cooling of the UF 6 . The XeF 6  reaction with U-235 enriched UF 5  produces a stable complex of UXeF 11 , which polymerizes to poly(pentafluoroxenonium(+1)-hexafluorouranate V), an intermediate product of the present process. The intermediate product may be thermally decomposed to form U-235 enriched UF 6  or U-235 enriched β-UF 5 .

CROSS -REFERENCE TO RELATED APPLICATION

This application is related to the subject matter disclosed in Application No. P 38 17 173.2 filed May 20th, 1988 in the Federal Republic of Germany, the entire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an improvement of the molecular laser process for uranium enrichment.

TECHNOLOGY REVIEW

In the molecular laser process for uranium enrichment uranium hexafluoride gas, composed of an isotope mixture of ²³⁵ UF₆ and ²³⁸ UF₆, is diluted in an inert gas and cooled by adiabatic supersonic expansion. The cooled isotope gas mixture is selectively excited by irradiation with laser light of one or a plurality of infrared wavelengths λ_(a) and is dissociated by simultaneous or subsequent exposure to one or further laser wavelengths λ_(d) in the infrared or ultraviolet spectral range (Equations 1a and 1b), See AIChE Symposium Series No. 221, Vol. 78, pages 61-70 (1982). ##STR1##

The U-235 enriched primary dissociation product (UF₅ monomers) polymerizes at temperatures below 100° C. into a crystalline solid body (β-UF₅) as the secondary product (Equations 2a and 2b), which can be separated from the circulating gas by subsequent physical separation processes such as filtration or the like.

    2UF.sub.5 →(UF.sub.5).sub.2                         ( 2a) ##STR2## where n≧2

From the publications by J. Lyman et al, J. Phys. Chem. 91 pages 4821-4827 (1987), and G. Grigor'ev et al., J. Sov. Chem. Phys. 3 (10), pages 2275-2281 (1986), it is known that UF₅ particles, particularly UF₅ monomers, are quickly fluorinated by fluorine atoms (Equations 3 and 4) to UF₆ (Equation 5 and 6). See also German Patent No. 2,651,122 and U.S. Pat. No. 4,212,716.

Equation 3:

Reverse reaction of the UF₅ monomer primary product from the photochemical UF₆ dissociation ##STR3##

Equation 4:

Reverse reaction of the UF₅ oligomer and polymer secondary product ##STR4##

Equation 5:

(Formal) isotope exchange of the primary product by UF₆ molecules in the gas phase ##STR5##

Equation 6:

Isotope exchange between the secondary product and UF₆ in the gas phase ##STR6##

Equation 7:

Adsorption of UF₆ at UF₅ monomers ##STR7##

Equation 8:

Adsorption of UF₆ at the (growing) UF₅ polymer ##STR8## where k₃ >K_(3') and k₄ >k₄ '.

As early as 1970 it was found that xenon hexafluoride reacts with solid crystalline UF₅ (produced by controlled fluorination of UF4 with elemental fluorine) to form a new, stable chemical compound, polypentafluoroxenonium(+1) hexafluorouranate V (XeF₅ ⁺ UF₆ -)_(n), see M. Bohinc, et al., J. Inorg. Nucl. Chem., 32, pages 1397-1400 (1970); M. Bohinc et al., J. Inorg. Nucl. Chem., 34, pages 2942-2946 (1972), M. Bohinc, et al., J. Inorg. Nucl. Chem., 34, pages 2938-2941 (1972). These publications characterize this compound crystallographically and physically as well as chemically. However, the above-described method of forming and separating UF₅ solids (equations 1a, 1b, 2a and 2b) has considerable drawbacks. Due to the possibility of parallel and reverse reactions of the photochemical dissociation product UF₅ (Equations 3 to 6) and of adsorption effects (Equations 7 to 8), the degree of enrichment achieved at the beginning of the process and rate of production of UF₅ monomers produced at the beginning of the process cannot be maintained.

SUMMARY OF THE INVENTION

The present invention provides an improved uranium isotope separation process following the molecular laser process in which uranium hexafluoride composed of an isotopic mixture of ²³⁵ UF₆ and ²³⁸ UF₆ is diluted in an inert gas and cooled by adiabatic expansion. The cooled isotopic mixture is selectively excited by irradiation with laser light of one or a plurality of infrared wavelengths and is photodissociated by simultaneous or subsequent exposure to laser light of another infrared wavelength or an ultraviolet wavelength to form a dissociation product. The dissociation product U-235 enriched UF₅, which may form UF₅ oligomer, still present in the gas phase, is brought into contact with gaseous xenon hexafluoride, causing it to react chemically and form ,a U-235 enriched complex compound UXeF₁₁. Subsequently UXeF₁₁ polymerizes to form a crystalline solid of poly(pentafluoroxenonium(+1)-hexafluorouranate V), an intermediate product which may then be separated from the gas mixture. The intermediate product may be thermally decomposed to form U-235 enriched UF₆ or U-235 enriched β-UF₅.

It is an object of the present invention to improve the selectivity in molecular laser processes for uranium enrichment.

It is another object of the present invention to improve the yield in molecular laser processes for uranium enrichment.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on the discovery that the U-235 enriched UF5 monomers, and U-235 enriched UF₅ oligomers present in the gas phase, which are primary products of the photochemical UF₆ dissociation, also react chemically with xenon hexafluoride to form a stable complex molecule which quickly polymerize to form a crystalline solid of poly(pentafluoroxenonium(+1)-hexafluorouranate V) (Equations 9 to 11). The XeF₆ reaction with UF₅ occurs faster than reactions of the UF₅ particles fluorine atoms or with UF₆ molecules.

It is an important aspect of the present invention that the UF₅ -XeF₆ reaction is utilized advantageously to form solid particles of poly(pentafluoroxenonium(+1)-hexafluorouranate V) in the molecular laser uranium enrichment process. In the present invention it has been found that if the appropriate amount of XeF₆ is added to the irradiated process gas stream, preferably immediately downstream of the laser light excitation step, stable complex molecules of UXeF₁₁ are formed which noticeably suppress selectivity reducing and yield reducing secondary reactions affecting the UF₅ monomers and at the growing UF₅ oligomers, respectively. See Equations 3 to 8. Thus the degree of enrichment, that is the selectivity, achieved at the beginning of the process is maintained, and the rate of production of U-235 enriched uranium produced by the process is maintained to a substantial degree as the process of the present invention continues, even after the formation of solid particles of poly(pentafluoroxenonium(+1)-hexafluorouranate V), which is an intermediate product of the present process.

The solid intermediate product produced by this process can then also be separated from the circulating gas by means of suitable physical separation processes, e.g. by filtration.

Equation 9:

Monomer reaction between XeF₆ and UF₅ ##STR9##

Equation 10:

Reaction between XeF₆ and UF₅ oligomers ##STR10##

Equation 11:

Polymerization of the complex monomer from UF₅ and XeF₆ to form the solid body ##STR11## where k₆ >k₃, k₄ and k'₆ >k'₃, k'₄

The isolated solid intermediate product can then either be reconverted to uranium hexafluoride by heating in a closed vacuum system (Equation 12a) or can be decomposed in a reversal of its formation by heating it in an inert gas stream to yield β-UF₅ and XeF₆ (Equation 12b), see M. Bohinc, et al., J. Inorg. Nucl. Chem., 34 pages 2938-2941 (1972).

Thermal decomposition of UXeF₁₁ : ##STR12## The XeF₄ produced in the statically thermal decomposition (Equation 12a) may be reacted with elemental fluorine to form additional XeF₆. The XeF₄, XeF₆ and any additional XeF₆ may be recovered by fractionated condensation.

The latter decomposition has the advantage that it permits the (very expensive) xenon hexafluoride to be continuously recovered quantitatively and returned to the process; additionally, this reversible pyrolysis produces extremely pure enriched β-UF₅, see M. Bohinc, et al., J. Inorg. Nucl. Chem., 34 pages 2938-2941 (1972).

It is understood that various other modifications will be apparent to and can readily be made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty that reside in the present invention, including all features that would be treated as equivalents thereof by those skilled in the art to which this invention pertains. 

What is claimed is:
 1. A uranium isotope separation process, comprising:diluting gaseous uranium hexafluoride composed of an isotopic mixture of ²³⁵ UF₆ and ²³⁸ UF₆ with an inert gas, cooling said diluted isotopic mixture by adiabatic expansion, selectively exciting said cooled isotopic mixture by irradiation with laser light of at least one infrared wavelength, photodissociating said excited isotopic mixture by simultaneous or subsequent exposure to laser light of another infrared wavelength or an ultraviolet wavelength, to produce a gas phase dissociation product selected from the group consisting of UF₅, an oligomer of UF₅, and mixtures thereof, reacting said gas phase dissociation product with gaseous xenon hexafluoride to form a UXeF₁₁ complex compound, polymerizing said UXeF₁₁ to form a solid of poly(pentafluoroxenonium(+1)-hexafluorouranate V), and separating said solid poly(pentafluoroxenonium(+1)-hexafluorouranate V).
 2. The process defined in claim 1, wherein the gaseous xenon hexafluoride is diluted with an inert gas.
 3. The process defined in claim 2, wherein the gaseous uranium hexafloride is diluted with the same inert gas.
 4. The process defined in claim 2, wherein the diluted gaseous XeF₆, with respect to the direction of flow of the UF₆, is fed in immediately downstream of the laser light excitation step.
 5. The poly(pentafluoroxenonium(+1)-hexafluorouranate V) process defined in claim 1, including decomposing the poly(pentafluoroxenonium(+1)-hexafluorouranate V) by irreversible thermal decomposition in a closed vacuum system to form enriched UF₆.
 6. The process defined in claim 5, wherein the irreversible thermal decomposition of poly(pentafluoroxenonium(+1)-hexafluorouranate V) forms xenon hexafluoride and xenon tetrafluoride.
 7. The process defined in claim 6 wherein the xenon hexafluoride and xenon tetrafluoride are recovered from the process gas stream together with excess xenon hexafluoride by fractionated condensation of the gas phase XeF₄ /UF₆ /XeF₆ /inert gas.
 8. The process defined in claim 6, including reacting the xenon tetrafluoride by fluoridation with elemental fluorine to form xenon hexafluoride.
 9. The process defined in claim 1, including forming enriched β-UF₅ and XeF₆ by means of reversible thermal decomposition of poly(pentafluoroxenonium(+1)-hexafluorourante V) in a stream of inert gas.
 10. The process defined in claim 9, including separating said XeF₆ and reacting said separated XeF₆ with additional gas phase dissociation product of said photodissociation. 